In software development, a given software system's ability to tolerate failures while still ensuring adequate quality of service—often generalized as resiliency—is typically specified as a requirement. However, development teams often fail to meet this requirement due to factors such as short deadlines or lack of knowledge of the field. Chaos engineering is a technique to meet the resilience requirement.

While overseeing Netflix's migration to the cloud in 2011,[2][3] Greg Orzell had the idea to address the lack of adequate resilience testing by setting up a tool that would cause breakdowns in their production environment, the environment used by Netflix customers. The intent was to move from a development model that assumed no breakdowns to a model where breakdowns were considered to be inevitable, driving developers to consider built-in resilience to be an obligation rather than an option:

"At Netflix, our culture of freedom and responsibility led us not to force engineers to design their code in a specific way. Instead, we discovered that we could align our teams around the notion of infrastructure resilience by isolating the problems created by server neutralization and pushing them to the extreme. We have created Chaos Monkey, a program that randomly chooses a server and disables it during its usual hours of activity. Some will find that crazy, but we could not depend on the random occurrence of an event to test our behavior in the face of the very consequences of this event. Knowing that this would happen frequently has created a strong alignment among engineers to build redundancy and process automation to survive such incidents, without impacting the millions of Netflix users. Chaos Monkey is one of our most effective tools to improve the quality of our services."[4]

By regularly "killing" random instances of a software service, it was possible to test a redundant architecture to verify that a server failure did not noticeably impact customers.

Chaos Monkey is a tool invented in 2011 by Netflix to test the resilience of its IT infrastructure.[2] It works by intentionally disabling computers in Netflix's production network to test how remaining systems respond to the outage. Chaos Monkey is now part of a larger suite of tools called the Simian Army designed to simulate and test responses to various system failures and edge cases.

The code behind Chaos Monkey was released by Netflix in 2012 under an Apache 2.0 license.[8][5]

The name "Chaos Monkey" is explained in the book Chaos Monkeys by Antonio Garcia Martinez:[9]

"Imagine a monkey entering a "data center", these "farms" of servers that host all the critical functions of our online activities. The monkey randomly rips cables, destroys devices and returns everything that passes by the hand [i.e. flings excrement]. The challenge for IT managers is to design the information system they are responsible for so that it can work despite these monkeys, which no one ever knows when they arrive and what they will destroy."

A tool that determines whether an instance is nonconforming by testing it against a set of rules. If any of the rules determines that the instance is not conforming, the monkey sends an email notification to the owner of the instance.

A "failure-as-a-service" platform built to make the Internet more reliable. It turns failure into resilience by offering engineers a fully hosted solution to safely experiment on complex systems, in order to identify weaknesses before they impact customers and cause revenue loss.[15]

To prepare for the loss of a datacenter, Facebook regularly tests the resistance of its infrastructures to extreme events. Known as the Storm Project, the program simulates massive data center failures.[16]

Inspired by AWS GameDays[17] to test the resilience of its applications, teams from Voyages-sncf.com participated in a Day of Chaos. Every 30 minutes, operators simulated failures in pre-production. Teams earned points based on detections, diagnoses, and resolutions. This type of gamified event helps to introduce development teams to the concept of resilience.[18]

Presented at the 2017 DevOps REX conference[19] the concept is presented on the site http://days-of-chaos.com in order to collect the other experiments.

The Chaos Toolkit was born from the desire to simplify access to the discipline of chaos engineering and demonstrate that the experimentation approach can be done at different levels: infrastructure, platform but also application. The Chaos Toolkit is an open-source tool, licensed under Apache 2, published in October 2017.[20]

The rapid pace of the DevOps methodology of software deployment makes it challenging to ensure a sufficient level of confidence in the face of frequent releases. A key element to address this is for monitoring and testing to be done throughout the development and release cycle. Integrating chaos engineering into the DevOps toolchain contributes to the goal of continuous testing.